It's also possible that Shallon genuinely doesn't realize she even HAS shardplate. She's gotten so used to her illusions that she might not have thought twice about what she had or hadn't drawn in the past, or that most of her illusions had little-to-no mass (with the exception of the 'army' she created for the battle, which could have been the mass of stormlight or a small amount of soulcasting, according to Jasnah.)
My point is, even if Shallon states emphatically that she doesn't think she has shardplate we shouldn't trust her. She is far from the most reliable narrator, especially regarding her own self/past.
Also, and I forgot this above, (back a bit to the op) she definitely attracted enough creation spren during the battle.
Additionally, the theory about default shard-plate color seems solid. We have, in OB a Dalinar Vision where he sees a Stoneward, climbing, weaing shardplate. Stoneward's gems are Topaz, and
Add this to Shallon's being Red/Garnet and we may have a coincidence. But then, in the Purelake vision....
It has been theorized (confirmed?) that this KR is a Dustbringer, and their stone is Ruby.
3rd time's a bit much to be a coincidence.